You probably checked it already... there is a (I think 80A) strip fuse in the glow plug relay.
I used B99 in my 740TD a lot without any issues. I never noticed any difference at all when I switched back and forth to/from B99, B50, B5 or regular diesel, except for the smell of course. I don`t think your B50 fuel should affect the glow system (same is not true on the filters though) but the pro d24t nuts will share their opinion as well. Currently (and all Winter long) I used B30 in mine as it is `Winter` but I will get back to B99 by April. I *never* had any starting or idling issues related to biodiesel fuel (not in cold temps either).
Btw, long ago in our 244GLE d24 I installed a small black pushbutton switch on the dashboard right next to the instrument cluster`s light adjuster switch. it worked as a protector against car-theft:
the car would not start without the glowplugs being on. I would love to have a manually operated glow plug system in mine so I could turn it on/off myself, this way avoiding headache and issues of them staying on for too long, or what`s worse : all times... until...
I mention here that I remember I also had the glow plug light come up unexpectedly, a few times while driving in the past
but that time I had my
ignition switch going downhill (thanks to some oily residue inside of it that got in by the key and covered everything in there resulting my ignition switch to not let me start the car at all). Any issues with that ign switch maybe? Good luck. I hope you sort it out soon!